I only did upgrades on my old MBP too, and always found it fine. I went from Leopard to Snow Leopard to Lion to Mountain Lion to Mavericks. Nothing serious went

I only did upgrades on my old MBP too, and always found it fine. I went from Leopard to Snow Leopard to Lion to Mountain Lion to Mavericks. Nothing serious went wrong at any point (though I did have to reinstall or update the odd program), and there was no noticeable deterioration in performance over time. There was the odd infuriating bug (I'm not convinced it's tested as thoroughly as Windows is...), but overall it was fairly trouble free and the smooth upgrade process played a part in my decision to buy another. It certainly wasn't the price, at any rate.